Gas turbines typically include a compressor section, a combustion section, and a turbine section. The compressor section pressurizes air flowing into the turbine. The pressurized air discharged from the compressor section flows into the combustion section, which is generally characterized by a plurality of combustors disposed in an annular array about the axis of the engine. Air entering each combustor is mixed with fuel and combusted. Hot gases of combustion flow from the combustion liner through a transition piece to the turbine section to drive the turbine and generate power. The turbine section typically includes a turbine rotor having a plurality of rotor disks and a plurality of turbine buckets extending radially outwardly from and being coupled to each rotor disk for rotation therewith. The turbine buckets are generally designed to capture and convert the kinetic energy of the hot gases of combustion flowing through the turbine section into usable rotational energy. In addition, the turbine section may also include an inner turbine casing and an outer turbine casing surrounding the inner turbine casing. As is generally understood, the inner turbine casing may be configured to encase the turbine rotor in order to contain the hot gases of combination. In doing so, a circumferential tip clearance is typically defined between the rotating buckets of the turbine rotor and an inner surface of the inner turbine casing.
During turbine operation, heat generated within the turbine results in thermal expansion of the turbine rotor and the inner turbine casing, which often causes variations in the tip clearances. For example, it may be the case that, while the turbine rotor expands consistently around its circumference, thermal expansion of the inner turbine casing may vary at different locations around its circumference (i.e., causing out-of-roundness of the casing). As a result, inadvertent rubbing may occur between the tips of the rotating buckets and the inner turbine casing, which can lead to premature failure of the buckets. Additionally, when excessive thermal expansion of inner turbine casing occurs, the tip clearances between the buckets and the inner turbine casing may become too large, thereby decreasing the overall efficiency of the gas turbine.
To facilitate optimizing turbine performance and efficiency and to minimize inadvertent rubbing between the bucket tips and the inner turbine casing, many gas turbines include active clearance control systems designed to supply a cooling fluid to the inner turbine casing, thereby promoting thermal contraction of the inner turbine casing to avoid tip rubbing. However, such clearance control systems typically require substantial pressure drops (regardless of whether the active control system is turned on or off) to facilitate cooling of the inner turbine casing. Thus, conventional clearance control systems are not as effective when the pressure drop through the system is required to be relatively low (e.g., when a gas turbine is operating at extreme temperatures and loads). Moreover, conventional clearance control systems typically require multiple air sources and are incapable of achieving deterministic heat transfer boundary conditions when the active control system is both on and off.
Accordingly, a clearance control system for gas turbines that addresses one or more of the problems identified above for conventional clearance control systems would be welcomed in the technology.